Temperature measurement and control is very important in injection molding. While making these measurements is relatively simple, most injection molding machines don't have enough temperature sampling points or lines.
On most injection molding machines, temperature is sensed by thermocouples. A thermocouple basically consists of two different wires joined at the end. If one end is hotter than the other, a tiny electrical signal will be generated; the more heated, the stronger the signal.
temperature control
Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, the desired temperature is set, and the sensor's display is compared to the temperature produced at the set point. In the simplest system, when the temperature reaches a set point, it is turned off, and the power is turned back on when the temperature drops. This system is called on/off control because it is either on or off.
Melt temperature
Melt temperature is important, and the temperature of the shot pot used is only a guideline. Melt temperature can be measured at the nozzle or using the air jet method. The temperature setting of the injection cylinder depends on the melt temperature, screw speed, back pressure, injection volume and injection cycle.
If you do not have experience with a particular grade of plastic, start with the lowest setting. For ease of control, the firing cylinders are divided into zones, but not all are set to the same temperature. If operating for a long time or at high temperature, please set the temperature of the first zone to a lower value, which will prevent the plastic from melting and shunting prematurely. Make sure the hydraulic oil, hopper closure, mold and injection cylinder are at the correct temperature before injection molding begins
Injection pressure
This is the pressure that causes the plastic to flow and can be measured with a sensor on the nozzle or hydraulic line. It has no fixed value, and the more difficult it is to fill the mold, the higher the injection pressure, and the injection line pressure is directly related to the injection pressure.
First stage pressure and second stage pressure
During the filling phase of the injection molding cycle, high injection pressures may be required to maintain the injection speed at the required level. Once the mold is filled, high pressure is no longer required. However, when injecting some semi-crystalline thermoplastics (such as PA and POM), the structure will deteriorate due to sudden pressure changes, so sometimes it is not necessary to use secondary pressure.
Clamping pressure
In order to counter the injection pressure, the clamping pressure must be used, do not automatically select the maximum value available, but consider the projected area and calculate a suitable value. The projected area of an injection molded part is the largest area seen from the direction of application of the clamping force.
For most injection molding situations, it's about 2 tons per square inch, or 31 meganewtons per square meter. However this is a low number and should be taken as a very rough rule of thumb, since, once the part has any depth, the sidewalls must be taken into account.
back pressure
This is the pressure that needs to be generated and exceeded before the screw moves back. Although high back pressure is beneficial to the uniform distribution of the colorant and the melting of the plastic, it also prolongs the return time of the middle screw, reduces the length of the fibers contained in the filled plastic, and increases the The stress of the injection molding machine; therefore, the lower the back pressure, the better. Under no circumstances should it exceed 20% of the injection pressure (maximum rating) of the injection molding machine.
nozzle pressure
Nozzle pressure is the pressure inside the nozzle. It is approximately the pressure that causes the plastic to flow. It has no fixed value, but increases with the difficulty of mold filling. There is a direct relationship between nozzle pressure, line pressure and injection pressure.
The nozzle pressure is about ten percent less than the injection pressure. In piston injection molding machines, the pressure loss can reach about 10%. In piston injection molding machines, the pressure loss can reach 50%.
Injection speed
This refers to the filling speed of the mold when the screw acts as a punch. When injecting thin-walled products, a high injection rate must be used so that the mold can be completely filled when the melt is not solidified, and a relatively smooth surface can be produced. Use a range of programmed rates of fire while filling to avoid defects such as jetting or trapped air. Injection can be performed under open-loop or closed-loop control systems.
No matter which injection speed is used, the speed value together with the injection time must be recorded on the record sheet. The injection time refers to the time required for the mold to reach the predetermined first-stage injection pressure, which is a part of the screw advance time.
